Update app.py

app.py

@@ -185,7 +185,8 @@ def toggle_custom_resize_slider(resize_option):
 def update_history(new_image, history):
     if history is None:
         history = []
-    history.insert(0, new_image)
+    if new_image is not None:
+        history.insert(0, new_image)
     return history
 
 css = """